Blow molding method for superplastic material and system

ABSTRACT

A method for blow molding a superplastic metal plate by in relation to time applying to it pneumatic pressure that is based on a maximum value of a strain rate of the superplastic metal plate as a set pattern of pneumatic pressure in relation to time when the metal plate is subjected to a high-speed blow molding after being heated to a desired temperature, comprising the steps of: entering data on a shape into which the metal sheet is to be blow molded and on properties of a material of the metal plate to store the data in a storage; determining a set pattern of a pneumatic pressure in relation to time from the entered data on the shape and the properties of the metal sheet; dividing the set pattern of the pneumatic pressure into an appropriate number of parts in relation to time; determining the values of parameters for controlling the pneumatic pressure for each part divided from the set pattern of the pneumatic pressure; and controlling the pattern of the pneumatic pressure using the determined values of the parameters for controlling the pneumatic pressure.

FIELD OF THE INVENTION

This invention relates to a method and a system for blow molding asuperplastic metal plate by applying to it a pattern of pneumaticpressure (a curve of the pneumatic pressure in relation to time) that isbased on the maximum value of the strain rate of the superplastic plateas a set pattern of pneumatic pressure in relation to time when thesuperplastic plate is subjected to a high-speed blow molding.

DESCRIPTION OF THE PRIOR ART

Recently a method has been developed for blow molding a superplasticplate such as an aluminum plate after it has been heated to a desiredtemperature. Since in this method the shape and the thickness of theplate vary as its formation proceeds, maintaining proper superplasticconditions relating to the strain rate is difficult, and there is adifficulty in achieving a stable formation. Thus, a system forcontrolling the pneumatic pressure of the blow molding such that themaximum strain rate of the plate is kept constant during its formationhas been discussed. In an example of this conventional control system,the maximum value of the strain rate is kept at a desired value (seePlasticity and Work [Journal of the Society of Japan Plastic Work] 31,1990, p.1128, by Akio Takahashi, et al., and Materials Science Forum,Vols. 304-306, 1990, p.777, by N. Suzuki et al.). Since in this controlsystem the strain rate of superplastic material is in an order of 10⁻³[1/s] and the obtained pattern of pneumatic pressure varies gradually,the strain rate can be easily controlled.

However, recently a high-speed blow molding has been developed, whereinthe strain rate of superplastic material is equal to or more than 10⁻²[1/s], which is faster by one order than the conventional strain rate,resulting in less time being required for blow molding it. Since in sucha high-speed blow molding the optimal pattern of the pneumatic pressureto keep the maximum value of the strain rate at a desired value variesgreatly, it became difficult to control the pattern of the pneumaticpressure as desired by a conventional blow molding machine.

The present invention has been conceived in view of such circumstances.The purpose of it is to provide a method and a system that canappropriately perform blow molding even if the strain rate of asuperplastic material is more than 10⁻² [1/s], wherein a pattern ofpneumatic pressure based on the maximum value of the strain rate isapplied to the material as a set pattern of the pneumatic pressure.

SUMMARY OF THE INVENTION

To the above end, in one aspect the method of the present invention ofblow molding superplastic material is a method for blow molding asuperplastic metal plate wherein pneumatic pressure in relation to timeand based on a maximum value of a strain rate of the superplastic metalplate is applied to the metal plate as a set pattern of pneumaticpressure in relation to time when the metal plate is subjected to ahigh-speed blow molding after being heated to a desired temperature,comprising the steps of: entering data on a shape into which the metalsheet is to be blow molded and on the properties of a material of themetal plate to store the data in a storage; determining a set pattern ofpneumatic pressure in relation to time from the entered data on theshape and the properties of the metal sheet; dividing the set pattern ofthe pneumatic pressure into an appropriate number of parts in relationto time; determining values of parameters for controlling the pneumaticpressure for each part divided from the set pattern of the pneumaticpressure; and controlling the pattern of the pneumatic pressure usingthe determined values of the parameters for controlling the pneumaticpressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing the embodiment of the method of thepresent invention.

FIG. 2 is a block diagram of the embodiment of the blow molding systemof the present invention.

FIG. 3 is a schematic representation of the embodiment of the blowmolding system of the present invention.

FIG. 4 is a graph showing a set pattern of a pneumatic pressure createdby setting the maximum value of the strain rate of a superplastic metalsheet to be blow molded as a set value.

FIG. 5 is a graph showing the measurements of the pneumatic pressurewhen it is controlled under inappropriate conditions of parameters of aPID control.

FIG. 6 is a table showing appropriate conditions of the parameters ofthe PID control for each of two time zones into which the set pattern ofthe pneumatic pressure is divided.

FIG. 7 is a graph showing the measurements of the pneumatic pressurewhen it is controlled under the appropriate condition of the parametersof the PID control.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The superplastic metal sheet used in this invention is an aluminum-alloysheet (this is a representative metal), or the like. In this preferredembodiment producing a thin form from a superplastic metal plate by blowmolding it is explained. The data on the shape of the thin form are thewidth, depth, etc., of a mold cavity. The data may be three-dimensionalCAD data. Further, the data on the properties of the material of thethin form are values representative of the properties of thesuperplastic material, including a strain-rate sensitivity exponent(m-value) and a K-value, which K-value is a constant representative ofthe stress level of the material. These values vary according tomaterials and their temperatures.

Generally, the property of a superplastic material is expressed in theequation a σ=K v ^(m), where σ is an equivalent stress, K is a constantrepresentative of the stress level of the material, v is its equivalentstrain rate, and m is its strain-rate sensitivity exponent.

Further, the temperature at which the metal plate is heated is, forexample, in the case of aluminum, its recrystallization temperature orsolidus temperature, i.e., 400-550° C., i.e., generally about 50-80% ofthe melting point of the material.

Further, the division of a pattern of the pneumatic pressure is todivide a curve of the pneumatic pressure into some parts in relation totime preferably into an area (a time zone) wherein the pressure variesgreatly and an area (a time zone) wherein the pressure varies gradually.Further, the controlling parameters are the parameters used forcontrolling the strain rate. There are three parameters in a PID controlthat are used in this embodiment i.e., a proportional band, integraltime, and derivative time.

The embodiment is now explained in detail by reference to FIGS. 1-7. Asin FIG. 2, the blow molding system for the present invention for blowmolding a superplastic material includes a computer 2, a conventionalinput device 1 for inputting conditions for forming in the computer 2,and a blow molding machine 3.

As is shown in FIG. 2, the computer 2 functions as a storage means 4 forstoring the inputted data on a shape into which the metal plate is to beformed and on the material of the metal plate, functions as a means 5for determining a set pattern of a pneumatic pressure in relation totime based on the data on the shape and the material of the metal platefrom the storage means 4, functions as a means 6 for dividing the setpattern of the pneumatic pressure into the appropriate number of partsin relation to time (time zones), functions as a means 7 for determiningvalues of parameters for controlling the pneumatic pressure for the partof the set pattern of the pneumatic pressure at each time zone, andfunctions as a control means 8 for controlling the pattern of thepneumatic pressure using the determined values of the parameters.

Further, as is shown in FIG. 3, the blow molding machine 3 includesupper and lower metal molds 9, 10 in which some electric heaters (notshown) are embedded, and means 11 for supplying compressed air to aplate P to blow mold it. The means 11 for supplying compressed airinclude a tank 12 for storing the compressed air, an electropneumaticproportional control valve in fluid communication with the tank 12, acheck valve 14 that connects the tank 12 to the upper and lower molds 9,10 so that a fluid can communicates between them, a pressure sensor 15,and a pipe 16. The pressure sensor 15 is electrically coupled to theelectropneumatic proportional control valve 13 via the computer 2.

The procedure to blow mold the plate P of an aluminum alloy by using theblow molding system arranged as explained above is now explained. First,a value of 100 mm for the diameter of the mold cavity, which is the dataon the molds (i.e., data on the shape into which the plate is formed),is entered in the computer 2 from the input device 1, and also the dataon the properties of the material, namely, the thickness of 1 mm, andthe strain-rate sensitivity exponent (m-value) 0.322, K-value 9.23×10⁻⁷,which represents the stress level, are entered in the computer 2 fromthe input device 1 (step S1). Then while the upper and lower molds 9, 10are being heated to 500° C., the plate P is set between them. Thecomputer 2 then determines a set pattern of the pneumatic pressure (thetheoretically set values of the pressure) in relation to time when themeans 5 is operated under the control of the computer 2 (step S2).

Generally, the blow molding has the pressure pattern wherein thepressure first rises and then descends.

Further, in the high-speed blow molding the pressure pattern becomesshorter in relation to time, and the pressure level becomes higher.Although in this embodiment the pressure rises to 0.5 MPa (5×10⁵ Pa)over 30 seconds and then drops gradually to 0.35 MPa during the next 60seconds, it may vary more greatly if other conditions than those forthis embodiment are selected.

To precisely control the pressure that varies greatly is very importantin controlling the strain rate and forming rate of the plate. In thisembodiment a PID control, which is the simplest feedback control, isused for controlling the pressure. In this PID control it is importantto determine the optimum values for the three parameters, namely, theproportional band, integral time, and derivative time. For fixed commandcontrols, the Ziegler Nichols method (the limiting sensitivity methodand the step response method) and the CHR method (the Chien, Hrones, andReswick method) have been proposed.

Since in the set pattern of the pneumatic pressure shown in FIG. 4 thepressure rises to 0.5 MPa and then drops to 0.35 MPa, in this embodimentthe values of the parameters of the PID control are obtained by thelimiting sensitivity method and by targeting a fixed command controlwherein the pressure in the cavities of the upper and lower molds 9, 10is kept, as a mean value, at a constant value of 0.3 MPa. The obtainedvalues of the parameters of the PID control are 4.8 for the proportionalband, 7 for the integral time, and 1 for the derivative time (PIDcondition 1). When the pressure of the cavity is kept at 0.3 MPa, theplate P is not set between the upper and lower molds 9, 10, and anoutlet for air provided in the lower mold 10 is plugged.

The results of controlling the set pattern of the pneumatic pressureusing the obtained values of the parameters are shown in FIG. 5 by thesolid lines. (This is a case wherein the set pattern of the pneumaticpressure is not divided in relation to time.) The measurements of thepressure (PID condition 1 in FIG. 5), in particular, of the pressurewhen it rises, differ greatly from the set values (the theoreticalpressure shown by a dotted line in FIG. 5). Thus the pressure of thecavity of the molds cannot be controlled under the PID condition 1. Thusstep S3 (shown below) is necessary.

After step S2, the computer 2 receives the data on the pneumaticpressure from the pressure sensor 15 of the blow molding machine 3 anddivides, in relation to time, the set pattern of the pneumatic pressureinto an appropriate number of parts (step S3). When it is divided,preferably as shown in FIG. 4, it is divided into two parts, namely thefirst time zone, from 0-30 seconds, and the second time zone, after 30seconds; the integral time for the part of the first time zone, wherethe pressure varies greatly, is made shorter to enhance its response;and the proportional band is made wider to restrain the tendency to bein a overshoot that may be caused by the enhancement of the response.Empirically, here the proportional band is made to be 4 times that ofPID condition 1, i.e., to be 19.2; the integral time is made to be aboutone-half, i.e., to be 4; and the derivative time remains as 1. Since thepressure varies gradually in the second time zone, the parameters of thePID condition 1 are used for that part. (See FIG. 6, PID condition 2.)

The computer 2 then determines the values of the parameters forcontrolling the pneumatic pressure for the parts divided from the setpattern of the pneumatic pressure (step S4), subsequently controls apattern of the pneumatic pressure based on the determined values of theparameter for controlling the pneumatic pressure, and enters the data onthe pattern of the pneumatic pressure in the electropneumaticproportional control valve (step S5). These steps are sometimesrepeated. As a result, the aluminum-alloy plate P is blow molded withthe pneumatic pressure that is generated along the set pattern as shownin FIG. 7.

The embodiment explained above is exemplary only, and the scope of theinvention is not limited to it. One skilled in the art will understandthat many variations can be made to the embodiment. Thus the inventionincludes such variations. Its scope will be defined by the followingclaims.

What we claim is:
 1. A method for blow molding a superplastic metalplate by in relation to time applying thereto pneumatic pressure that isbased on a maximum value of a strain rate of the superplastic metalplate as a set pattern of pneumatic pressure in relation to time whenthe metal plate is subjected to a high-speed molding where the strainrate is more than 10⁻²(1/s), after being heated to a desiredtemperature, comprising the steps of: entering data in a storage meanson a shape into which a metal sheet is to be blow molded and onproperties of a material of the metal sheet to store the data in thestorage means; determining a set pattern of a pneumatic pressure inrelation to time from the entered data on the shape and the propertiesof the metal sheet; dividing the set pattern of the pneumatic pressureinto an appropriate number of parts in relation to time, wherein a firstpart divided from the set pattern of the pneumatic pressure is a firstpattern area in which the pneumatic pressure varies relatively steeply,and wherein the remaining part divided from the set pattern of thepneumatic pressure is a second pattern area, following the first patternarea, in which the pneumatic pressure varies gradually compared with thefirst pattern area; determining values of parameters for controlling thepneumatic pressure for each part divided from the set pattern of thepneumatic pressure, wherein the values of the parameters are determinedby changing the values of the parameters of a proportional plus integralplus derivative control for the parts divided from the pattern of thepneumatic pressure; and controlling the pattern of the pneumaticpressure using the determined values of the parameters.
 2. A system forblow molding a superplastic metal plate by in relation to time applyingthereto a pneumatic pressure that is based on a maximum value of astrain rate of the superplastic metal plate as a set pattern ofpneumatic pressure in relation to time when the metal plate is subjectedto a high-speed blow molding where the strain rate is more than10⁻²(1/s), after being heated to a desired temperature, comprising:means for entering data on a shape into which a metal sheet is to beblow molded and on properties of a material of the metal sheet; meansfor storing the data; means for determining a set pattern of a pneumaticpressure in relation to time from the data on the shape and theproperties of the metal sheet stored in the storing means; means fordividing the set pattern of the pneumatic pressure into an appropriatenumber of parts in relation to time; means for determining values ofparameters for controlling the pneumatic pressure for each part dividedfrom the set pattern of the pneumatic pressure; and means forcontrolling the pattern of the pneumatic pressure using the determinedvalues of the parameters.
 3. The system of claim 2, wherein theappropriate number of parts divided from the set pattern of thepneumatic pressure includes a first pattern area in which the pneumaticpressure varies relatively steeply and a second pattern area in whichthe pneumatic pressure varies gradually compared with the first patternarea.